The separation of one or more gases from a complex multicomponent mixture of gases is necessary in a large number of industries. Such separations currently are undertaken commercially by processes such as cryogenics, pressure swing adsorption and membrane separations. In certain types of gas separations, membrane separations have been found to be economically more viable than other processes.
In a pressure driven gas membrane separation process, one side of the gas separation membrane is contacted with a complex multicomponent gas mixture and certain of the gases of the mixture permeate through the membrane faster than the other gases. Gas separation membranes thereby allow some gases to permeate through them while serving as a barrier to other gases in a relative sense. The relative gas permeation rate through the membrane is a property of the membrane material composition. It has been suggested in the prior art that the intrinsic membrane material selectivity is a combination of gas diffusion through the membrane, controlled in part by the packing and molecular free volume of the material, and gas solubility within the material. It is highly desirable to form defect free dense separating layers in order to retain high gas selectivity.
The preparation of commercially viable gas separation membranes has been greatly simplified with asymmetric membranes. Asymmetric membranes are prepared by the precipitation of polymer solutions in solvent-miscible nonsolvents. Such membranes are typified by a dense separating layer supported on an anisotropic substrate of a graded porosity and are generally prepared in one step. Examples of such membranes and their methods of manufacture are shown in U.S. Pat. Nos. 4,113,628; 4,378,324; 4,460,526; 4,474,662; 4,485,056; and 4,512,893. U.S. Pat. No. 4,717,394 shows preparation of asymmetric separation membranes from selected polyimides.
A shortcoming of asymmetric gas separation membranes concerns the stability of these membranes under end use environmental conditions because asymmetric membranes are typically composed of homogeneous materials. That is to say, the dense separating layer and the porous substrate layer of the membrane are compositionally the same.
For some gas separations, such as acid gas separations, it has been found advantageous in the prior art to employ separating membranes comprising materials which have high intrinsic acid gas solubility. However, asymmetric membranes prepared from materials with high acid gas solubilities tend to plasticize and undergo compaction under acid gas separation end use conditions. In addition, asymmetric membranes may be plasticized and compacted due to components such as water which may be in the gas mixtures to be separated. As a result, asymmetric gas separation membranes prepared from hydrophilic materials may be adversely affected under such conditions.
Composite gas separation membranes typically have a dense separating layer on a preformed microporous substrate. The separating layer and the substrate are usually different in composition. Examples of such membranes and their methods of manufacture are shown in U.S. Pat. Nos. 4,664,669; 4,689,267; 4,741,829; 2,947,687; 2,953,502; 3,616,607; 4,714,481; 4,602,922; 2,970,106; 2,960,462; and 4,713,292, as well as in Japanese 63-218213.
U.S. Pat. No. 4,664,669 discloses hollow fiber composite membranes of a dense, polyorganosilane polymer and an ultra-microporous layer supported on a porous substrate. U.S. Pat. Nos. 4,689,267 and 4,714,481 show hollow fiber composite membranes that include a dense coating of a poly(silylacetylene) on a porous hollow fiber support. U.S. Pat. No. 4,741,829 shows bicomponent, melt-spun hollow fiber membranes. U.S. Pat. No. 4,826,599 shows forming hollow fiber composite membranes by coating a porous hollow fiber substrate with a solution of membrane forming material, and coagulating the membrane forming material. Japanese patent application 63-218,213, published Sept. 12, 1988, shows coextruding two solutions of polysulfone to form a composite membrane. U.S. Pat. No. 2,947,687 shows composite membranes that include a thin layer of ethyl cellulose. U.S. Pat. No. 2,953,502 shows thin, non-porous plastic membranes. U.S. Pat. No. 2,970,106 shows composite membranes that include modified cellulose acetate-butyrate. U.S. Pat. No. 3,616,607 shows dense polyacrylonitrile film onto a nonporous preforms. U.S. Pat. No. 4,602,922 shows a polyorganosiloxane layer between a porous substrate and the dense separation layer of a composite membrane. U.S. Pat. No. 4,713,292 melt-spun, multi-layer composite hollow fiber membranes. U.S. Pat. No. 2,960,462 shows a non-porous selective film laminated onto a thicker, non-porous permeable film.
Composite gas separation membranes have evolved to a structure of an ultrathin, dense separating layer supported on an anisotropic, microporous substrate. These composite membrane structures can be prepared by laminating a preformed ultrathin dense separating layer on top of a preformed anisotropic support membrane by a multistep process. Examples of such membranes and their methods of manufacture are shown in U.S. Pat. Nos. 4,689,267; 4,741,829; 2,947,687; 2,953,502; 2,970,106; 4,086,310; 4,132,824; 4,192,824; 4,155,793; and 4,156,597.
U.S. Pat. No. 4,086,310 shows preparation of composite membranes from supported, ultra-thin, dense polycarbonate. U.S. Pat. Nos. 4,132,824 and 4,192,842 show ultra-thin dense 4-methylpentene film composite membranes. U.S. Pat. No. 4,155,793 shows composite membranes that include an ultra-thin, dense film on a porous substrate. U.S. Pat. No. 4,156,597 shows a composite membrane that includes an ultra-thin, dense polyetherimide separation layer.
Composite gas separation membranes are generally prepared by multistep fabrication processes. Typically, the preparation of composite gas separation membrane requires first forming an anisotropic, porous substrate. This is followed by contacting the substrate with a membrane-forming solution. Examples of such methods are shown in U.S. Pat. Nos. 4,826,599; 3,648,845; and 3,508,994.
U.S. 3,508,994 shows contacting a porous substrate with a membrane forming solution. U.S. Pat. No. 3,648,845 shows coating a porous substrate with a buffer layer followed by solution casting a separating layer of cellulose acetate. Dip coatinq a polymer solution onto the substrate also may be employed. Examples of such methods are shown in U.S. Pat. Nos. 4,260,652; 4,440,643; 4,474,858; 4,528,004; 4,714,481; and 4,756,932. U.S. Pat. No. 4,260,652 dip coats a polymer onto a substrate. U.S. Pat. No. 4,440,643; 4,474,858; and 4,528,004 show composite polyimide membranes formed by coating a substrate. U.S. Pat. No. 4,714,481 dipcoats polyacetylene onto a substrate to form a composite membrane. U.S. Pat. No. 4,756,932 shows forming composite hollow fiber membranes by dip coating.
The multistep fabrication processes of the prior art tend to be expensive and time consuming. In addition, the composite membranes produced by these multistep processes can experience failure and poor performance due to defects in the substrate and separating layer. A need therefore exists for a membrane and a process of manufacture which avoids the above shortcomings of the prior art membranes and processes.